Ultra-wideband (UWB) communications systems are normally defined as carrier-less communications systems wherein the bandwidth of the signal being transmitted, fB, is greater than or equal to 0.20 fc, where fc is the center frequency of the signal being transmitted. Additionally, the UWB communications system should have a minimum bandwidth of 500 MHz. Note that the definition for UWB communications systems and devices is as defined by the Federal Communications Commission (FCC) of the United States. UWB communications systems have been around for a great number of years, and the majority of them fall under a single type of system, they modulate a stream of short-duration pulses (with an approximate duration which ranges from 0.2 nanoseconds (ns) to 2 ns), either in time (pulse position modulation (PPM)), amplitude (pulse amplitude modulation (PAM)), or phase angle (bi-phase modulation).
The FCC, in Report Order 02-48 released in February of 2002, has specified a set of spectral allocation, technical standards, and operating restrictions for several different types of UWB devices. For example, in the Report Order, the FCC specifies that indoor UWB devices may operate within a frequency range of 1.9 to 10.6 GHz while hand-held UWB devices may operate within a frequency range of 3.1 to 10.6 GHz. Within the permitted frequency ranges, the FCC also places a limit upon maximum transmit power.
In order to maximize data transmission rates and operating range, designers of UWB systems can simply transmit at the maximum permitted transmit power as specified by the FCC. An ideal pulse that can be used to transmit a signal in a UWB system that can make use of the entire permitted frequency band and that can fit the specified power spectral constraints would be a rectangular shaped signal in the frequency domain. Unfortunately, a rectangle in the frequency domain corresponds to an infinite duration sinc function in the time domain. Due to its infinite length in time and a large number of taps required to generate the signal the rectangular shaped signal is infeasible to implement.
A commonly used signal in UWB is known as the ‘chirp.’ Two different implementations of the chirp signal have been disclosed in previous patents. A first is called the Gaussian weighted sinusoidal function and the second is called the inverse-exponentially weighted sinusoidal function. The two versions of the chirp signal offer a good approximation of the rectangular shaped signal in the frequency domain, and therefore, are good candidates for use in a UWB system.
One disadvantage of the prior art is that there are signal components of the chirp signals exist at every frequency within the transmit frequency band. Therefore, if there are wireless communications systems operating within the general vicinity of the UWB system, the transmission of the chirp signals could pose interference problems. Furthermore, the transmissions from the wireless communications systems can also interfere with the communications of the UWB system.
A second disadvantage of the prior art is that in order to provide a good approximation of the rectangular shape, a relatively large number of taps may be needed. The large number of taps can make it difficult to actually implement the pulse in a UWB system.